Method for washing and protecting photographic silver images



United States Patent 3,533,790 METHOD FOR WASHING AND PROTECTING PHOTOGRAPHIC SILVER IMAGES Howard C. Haas, Arlington, Mass., assignor to Polaroid Corporation, Cambridge, Mass., a corporation of Delaware No Drawing. Filed Sept. 15, 1965, Ser. No. 487,596 Int. Cl. G03c 5/26, 5/54 US. Cl. 96-29 2 Claims ABSTRACT OF THE DISCLOSURE The stability of silver images produced by diffusion transfer photographic techniques is enhanced by applying to the silver print alkaline solution comprising zinc oxide, ammonia and an acid polymer, e.g., acid-substituted cellulose and vinyl polymers, and drying the solution to form a protective coating thereon. Preferred compositions contain, in addition to the above ingredients, water and a water-miscible organic solvent.

This invention relates to the protecting of photographic images by forming protective coatings therefor, to the washing and protecting of photographic silver images by removing residual processing reagents therefrom and forming protective coatings therefor, and to the photographic products of such processes.

Objects of the present invention are: to provide a process for increasing the stability of a photographic silver image with a novel composition comprising water for washing the image when the composition is applied and a dispersion of such materials for forming a coating upon the image when the composition is dried; to provide, as a novel product, a protected photographic image produced by such a process; and to provide for use in a process of the foregoing type, a composition comprising a mixture of an acid polymer, zinc oxide, and ammonia.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the process involving the several steps and the relation and order of one or more of such steps with respect to each of the others, and the composition and product possessing the features, properties and relation of elements which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the appended claims.

For a fuller understanding of the nature and objects of of the invention, reference should be had to the following detailed description.

The process of the present invention is particularly useful for washing and protecting thin photographic silver images prepared in accordance with well-known diffusion transfer processes, for example, as described in US. Pat. No. 2,719,791, issuedto Edwin H. Land on Oct. 4, 1955. An extremely thin, photographic silver image of the foregoing type ordinarily retains traces of the photographic reagents with which it has been processed, and the continued presence of which may adversely affect its stability. For example, silver may be oxidized by sulphur from the residue of sodium thiosulfate which has been employed as a solvent. Or, the silver may be oxidized by numerous oxidizing agents present in the atmosphere. Also, the image may comprise traces of silver ion in equilibrium with the metallic silver; where the image is subjected to materials which form insoluble silver residues, such as sulfides, they react with the silver ion, causing more metallic silver to shift to the ionic state, ultimately destroying the silver image entirely. Furthermore, traces of unoxidized developer, if oxidized by at- 3,533,790 Patented Oct. 13, 1970 mospheric oxygen, may discolor the highlights of the image.

It has been proposed, in order to improve the stability of such an image, to coat it with a composition comprising an aqueous solution of a film-forming material. The water acts to wash traces of the photographic reagents from the image and to so distribute the film-forming material that it provides the image with a protective coating when the dispersion is dried. Processes of this type are more fully described in US. Pat. 2,719,791, cited above.

In accordance with the present invention, a preferred composition for this purpose comprises a mixture of compatible components which together possess properties not possessed by the components themselves. These components includes zinc oxide, a polymeric material having acid functional groups thereon, and ammonia. Preferably, the composition also comprises water and a watermiscible organic solvent. Such a composition, when used as a protective coating for photographic silver images of the type described, has been fround to provide the following unusual combination with respect to the images: substantial impermeability to water, elemental sulphur, and hydrogen sulfide, for example, from sulphur-containing materials with which the composition may come into contact; image stability throughout wide temperature and humidity ranges; resistance to generalized oxidation of silver; resistance to the effects of prolonged solar radiation; optical clarity; and non-tackiness.

Without intending to be bound by theoretical considerations, it is believed that the following factors and reactions are at least in part responsible for the unexpectedly superior image-protecting properties of the coating compositions of this invention. It is believed that in the coating composition, the zinc oxide and ammonia react to form a zinc ammonium complex which complex, in turn, enters a state of equilibrium with zinc hydroxide and ammonia. These reactions may be represented as follows:

When such a composition is applied as a coating to photographic images and allowed to dry, ammonia evaporates, thereby generating additional zinc hydroxide. As zinc hydroxide is formed, however, it reacts with the available acid groups on the polymeric material. The net effect is that the polymers are cross-linked and hardened, thus providing a superior photographic print coating.

It is to be noted that the foregoing cross-linking system is dependent upon the use of zinc oxide. Soluble metal salts such as those employed in other photographic coatings known heretofore, e.g., zinc acetate, are unable to undergo the above type of reaction and accordingly, do not provide the superior results obtainable when zinc oxide is combined with ammonia and an acid polymer as in the compositions described herein.

Where, for example, the acid polymer is a phthalaldehydic acid partial acetal of polyvinyl alcohol, the crosslinked product from which ammonia, water, and solvent had evaporated, would be composed of the recurring unit having the following structure:

The organic solvent, examples of which are low molecular weight alcohols such as methanol, ethanol and isopropanol, dioxane, and low molecular weight ketones such as acetone and methylethyl ketone, acts in conjunction with the water and ammonia to effect dissolution of the polymeric material (which may be insoluble in water alone) and to impart to the composition an overall quickdrying character.

The zinc oxide serves not only as a cross-linking agent but also provides a heavy metal salt which forms an insoluble colorless sulfide, in accordance with the teachings of US. Pat. 2,866,705, issued Dec. 30, 1958 to Edwin H. Land and Meroe M. Morse. It should be noted that while zinc oxide is the salt of choice for purposes of the instant invention, cadmium oxide is also fully operative in that cadmium forms insoluble sulfides which are nearly as colorless as those of zinc, it forms complexes with ammonia, it can provide the basis of an equilibrium similar to that discussed above for zinc, and can thus effect crosslinking of the acid polymer.

The acid polymers within the scope of this invention are preferably cellulose or vinyl film-forming polymers having acid radicals, preferably carboxylic (COOH') or sulfonic (SO H) radicals attached thereto. It is also within the scope of this invention to employ polymers with carboxylic acid anhydride groups, at least some of which have been converted to free carboxyl groups. As examples of acid polymeric materials which are operative in this invention, mention may be made of cellulose acetate hydrogen phthalate; cellulose acetate hydrogen glutarate; cellulose acetate hydrogen succinate; ethyl cellulose hydrogen succinate; ethyl cellulose acetate hydrogen succinate; cellulose acetate hydrogen succinate hydrogen phthalate; ether and ester derivatives of cellulose modified with sulfoanhydrides, e.g., ortho sulfobenzoic acid; polystyrene sulfonic acid; carboxymethyl cellulose; polyvinyl hydrogen phthalate; polyvinyl acetate hydrogen phthalate; polyacrylic acid; acetals of polyvinyl alcohol with carboxyor sulfo-substituted aldehydes, e.g., o, m, or p-benzaldehyde sulfonic acid or carboxylic acid or formyl alkanoic or alkane sulfonic acid; partial esters of ethylene/maleic anhydride copolymers; and partial esters of methylvinyl ether/maleic anhydride copolymers. In particular, aldehydic acid substituted partial acetals of polyvinyl alcohol have been found to be especially well suited to the compositions of this invention. The use of these latter materials are disclosed and claimed in copending application of Howard C. Haas, Ser. No. 487,597, filed concurrently herewith.

These latter polymeric materials may be prepared by reacting a low molecular weight polyvinyl alcohol, preferably one which is completely hydrolyzed, with a compound having the formula RCHO, Where R has the same meaning as is given above, in the presence of an acid catalyst.

Alternatively, the polymers may be prepared by reacting polyvinyl alcohol with an ester having the formula:

where m is an integer from 1-4, inclusive, and each R is an alkyl radical, or the corresponding acetals of the above esters, and subsequently hydrolyzing the ester group to the corresponding acid. In the preferred polymeric formulations, a total of 28% to 42% of the hydroxyl groups on the polyvinyl alcohol chain are substituted with the acid function groups. It has been found that if the substitution is as low as 20%, the resulting composition may be undesirably viscous. The substitution can theoretically be as high as about 90% but there are substantially no advantages to be gained thereby.

The quantity of ammonia to be incorporated in the compositions of this invention is not critical, other than that it be suificient to render the final pH of the composition basic. In a preferred embodiment the pH of the composition is about 8 to 14, the quantity of ammonia being regulated to achieve this. The use of alkaline print coaters is described and claimed in the copending application of Edwin H. Land and Meroe M. Morse, Ser. No. 487,578, filed concurrently herewith.

It is also sometimes desirable to add an additional polymeric material to the coating composition to further enhance its protective properties. The preferred material for this purpose is a hydantoin formaldehyde condensation polymer, such as the dimethyl hydantoin formaldehyde polymer described in US. Pat. No. 2,874,045, issued Feb. 17, 1959 to Edwin H. Land, for Process of Washing and Protecting Photographic Silver Images.

The compositions within the scope of this invention may be prepared by dissolving the acid polymer in the Water, organic solvent, and ammonia at a temperature ranging from room temperature to 40 C. under a reflux condenser to prevent evaporation of the solvent. Thereafter, the zinc oxide, and optionally, the hydantoin formaldehyde polymer are stirred into the mixture. A preferred composition comprises the foregoing ingredients in the following proportions:

Acid polymer-8 to 25 g.

Water50* to 70 cc.

Organic solvent30 to 50 cc.

Zinc oxide-1 to 10 g.

Hydantoin formaldehyde condensation polymer-5 to Ammoniasuificient to dissolve the polymer and give a final pH of about 9 to the final composition.

The following non-limiting example illustrates the superior photographic silver image coatings which are obtained with the compositions of this invention.

A composition for washing and protecting photographic silver images was prepared by combining the following materials to provide a uniform solution:

Phthalaldehydic acid partial acetal of polyvinyl alcohol- Dimethyl hydantoin formaldehyde condensation polymer13.5 g.

Zinc oxide-1.44 g.

Water-50 cc.

Isopropyl alcohol-30 cc.

Ammonia (29.4% aqueous solution)10 cc.

The final pH was about 9.

Photographic silver transfer images were prepared-by exposing photosensitive silver halide elements to a step- Wedge, and developing the latent images formed thereby by spreading between the exposed photosensitive elements and superposed image-receiving elements a processing composition containing a viscous aqueous solution of a silver halide developing agent, a silver halide solvent, and an alkali. The respective elements were maintained in superposed relation for a predetermined period, during which time the exposed silver halide was reduced to silver in the photosensitive elements. The unreduced silver halide formed a water-soluble complex silver salt which diffused through the layer of composition to the image-receiving elements where, upon being reduced to silver, it formed a silver print. At the end of the predetermined period, the photosensitive elements, together with the layer of composition, were stripped from the image receptive elements. The photographic silver images so prepared were swabbed by means of an absorbent applicator with the coating solution prepared as described above. The prints so treated dried rapidly under ordinary atmospheric conditions, after which they were subjected to the following tests:

(a) The prints were allowed to remain in closed chambers at a temperature of 100 F. and relative humidity for a period of about 40 hours.

(b) The prints were suspended over water at 100 F. for a period of about days.

(c) The prints were subjected to dry air at 100 F. and 20% relative humidity for a period of about 16 hours, to test for cracking of the print layer.

(d) The prints were subjected to 4 and 8 hour periods in an atmosphere containing hydrogen sulfide (provided by an aqueous solution of sodium sulfide) at room temperature.

(e) The prints were stacked face to face at relative humidities of 20% and 80%; a weight was placed on the stack.

After all of the foregoing tests, it was observed that in none of the prints was there any deterioration of the silver image, nor was there any loss in density in the light steps of the image. The prints were non-tacky, as evidenced by the fact that in test (e), they exhibited no tendency to adhere to one another. Further, there was no evidence of discoloration of the image.

The polymer used in the foregoing example was prepared as follows:

Phthalaldehydic acid partial acetal of polyvinyl alcohol polymer composed of the recurring unit 88 grams (2 moles) of low viscosity, low molecular weight polyvinyl alcohol having less than 1% acetate were dissolved in 500 cc. of water and cooled. 150 grams (1 mole) of phthalaldehydic acid were added with stirring and the mixture was heated at 50 C. under nitrogen. 1 cc. of concentrated sulfuric acid was added. The polymer separated as a soft, rubbery mass. After four hours, the liquid was decanted and the product washed in hot water. It was further purified by precipitation from ethanol-water (containing 1 gram of potassium acetate) into water, washed in water and dried. The final product was predominantly the phthalaldehydic acid partial acetal of polyvinyl alcohol, 'with a small amount of the corresponding phthalide; the composition was confirmed by infrared analysis.

Examples of photographic materials useful in the production of the photographic silver images to which the preferred composition may be most advantageously applied are described in detail in Pat. No. 2,543,181, issued in the name of Edwin H. Land on Feb. 27, 1951 for Photographic Product Comprising a Rupturable Container Carrying a Photographic Processing Liquid, and in Pat. No. 2,647,056, issued in the name of Edwin H. Land on July 28, 1953 for One-Step Photographic Transfer Process.

Preferably, the composition of the present invention is 6 i 1 applied to a photographic silver image to be washed and protected by means of an absorbent applicator composed, for example, of flannel, cotton batting, or cellulose sponge charged with the composition. When the face of the image is swabbed with such an applicator, residual reagents in the image are dissolved and, for the most part, transferred into the applicator, and the image becomes coated with a thin layer of the dispersion. The dispersion can then be dried to form a protective coating.

The preferred composition referred to above, after being swabbed by means of such an absorbent applicator onto a thin photographic silver image of the above-described type, rapidly dries under ordinary atmospheric con ditions to form a coating having excellent protective properties for the image.

Since certain changes may be made in the above process, composition and product without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description shall be inter preted as illustrative and not in a limiting sense.

What is claimed is:

1. In a process for washing and protecting a diffusion transfer photographic silver print containing a residue of the reagents with which it has been processed, the step of applying to one face thereof, a thin layer of an alkaline composition comprising zinc oxide, ammonia, and an acid film-forming polymer selected from the group consisting of cellulose and vinyl polymers containing groups selected from carboxylic acid and sulfonic acid groups, said alkaline composition has a pH between 8 and 14; and for-m ing a protective coating on said print by removing ammonia by drying said layer.

2. The process of claim 1 wherein said acid polymer is a phthalaldehydic acid substituted partial acetal of polyvinyl alcohol.

References Cited UNITED STATES PATENTS 2,759,825 8/1956 Land 9629 2,830,900 4/1958 Land et al 9629 2,874,045 2/ 1959 Land 9629 2,956,877 10/1960 Land et a1 9629 2,979,477 4/1961 Land 260-294 3,174,858 3/1965 Van Hoof et al 9629 3,325,283 6/1967 Barstow et al. 963 2,794,740 6/1957 Land et al 9629 2,866,705 12/1958 Land et a1 9629 7 OTHER REFERENCES Condensed Chemical Dictionary 6th Edition, Reinhold, p. 1247.

NORMAN G. TORCHIN, Primary Examiner M. F. KEL'L'EY, Assistant Examiner U.S. Cl. X.R. 9650 

